pango: use "ifdef" instead of "if defined"
pango: remove duplicated definition

It exists in rbpangoprivate.h.
glib2: remove needless RubyGems fallback

We already dropped Ruby 1.8.
pango: require rcairo
pango: suppress a deprecated function used warning

    rbpangocairo.c: In function ‘rg_create_context’:
    rbpangocairo.c:91:5: warning: ‘pango_cairo_font_map_create_context’ is deprecated (declared at /usr/include/pango-1.0/pango/pangocairo.h:79): Use 'pango_font_map_create_context' instead [-Wdeprecated-declarations]
         return GOBJ2RVAL_UNREF(pango_cairo_font_map_create_context(_SELF(self)));
         ^
pango: drop Pango < 1.8
pango: remove trailing spaces
gtk2 gtk-demo: fix file loading error when running on other directory